Combined cycle power plants are combined power-generating plants in which a gas turbine and a steam turbine are combined. The combined cycle power plants generate power by driving the gas turbine with combustion gas generated by supplying fuel gas to compressed air for combustion, and generate power by driving the steam turbine with steam generated by feeding high-temperature flue gas discharged from the gas turbine to a heat recovery steam generator and generating steam using a heating unit of the heat recovery steam generator.
Such combined cycle power plants are provided with a fuel gas heating apparatus for heating fuel gas with high-temperature water from a heat recovery steam generator. The fuel gas heating apparatus includes a large number of tubes disposed in the heating apparatus body. Fuel gas is supplied to the heating apparatus body while high-temperature water is supplied to the large number of tubes, so that heat exchange is performed between the fuel gas and the high-temperature water.
In the fuel gas heating apparatus, long-term use can cause a leak (leakage) of fuel gas or high-temperature water due to breakage of a tube. Thus detection of tube leaks is performed during operation or during halts. During the operation of the fuel gas heating apparatus, a tube leak is detected conventionally by detecting the amount of water leaking from a tube to the heating apparatus body because the feedwater pressure is higher than the pressure of the fuel gas. During a halt of the fuel gas heating apparatus, a tube leak is detected according to a decreasing tendency of the feedwater pressure because the pressure of the fuel gas cooled is higher than the feedwater pressure.
A method for detecting tube leaks in a fuel gas heating apparatus is described, for example, in Patent Literature 1. An apparatus and a method for detecting breakage of a tube in a gas heating apparatus described in Patent Literature 1 determine that the tube is broken when a condition in which a pressure differential between the pressure in the tube on the container outlet side and the pressure of gas passing through the container is substantially zero continues for a specified time or more after stopping the supply of a fluid to the tube and discharging the fluid to the outside of the tube.